Amedeo Avogadro

The first hypothesis that presents itself to the mind, and which seems to me to be the most natural, is that the number of integral molecules in any gas is always the same for the same volume, or for equal volumes, under the same conditions of temperature and pressure.

It must be admitted that there is a very simple relation between the volumes of gaseous substances and the numbers of integral molecules or atoms composing them.

The integral molecules of any gas are always at the same distance from each other, when the gases are under the same conditions of temperature and pressure.

We must admit that the number of molecules in equal volumes of different gases, at the same temperature and pressure, is the same.

The hypothesis of equal numbers of molecules in equal volumes is, I believe, the only one which can reconcile the results of Gay-Lussac on the combination of gaseous substances with the atomic theory of Dalton.

It is necessary to distinguish between the molecule of a simple body and the molecule of a compound body.

The elementary molecules of bodies may be composed of a certain number of integral molecules, or even of a single one.

The integral molecules of simple bodies are not necessarily simple atoms, but may be composed of two or more atoms united together.

This distinction between the molecule and the atom is of the utmost importance for the progress of chemistry.

The number of molecules in equal volumes of gases is a constant, regardless of the nature of the gas.

The relative masses of the elementary molecules are proportional to the densities of the gases.

This hypothesis, if it is admitted, furnishes a means of determining the relative masses of the elementary molecules of bodies.

The simplicity of the relations which exist between the volumes of gaseous substances when they combine, leads us to admit that there is a similar simplicity in the relations between the numbers of their molecules.

The integral molecules of simple gases are composed of two atoms.

This idea, which at first sight may appear somewhat paradoxical, is nevertheless supported by strong analogies.

The law of Gay-Lussac on the combination of gaseous substances by volume, and the hypothesis of Dalton on the atomic composition of bodies, can only be reconciled by admitting that the integral molecules of simple gases are composed of two atoms.

The number of molecules in a given volume of gas is independent of the nature of the gas.

The relative masses of the molecules of different gases are proportional to their densities.

This hypothesis allows us to determine the relative masses of the elementary molecules of bodies, and the proportions according to which they enter into their combinations.

The integral molecules of compound bodies are formed by the union of elementary molecules.